Conventional servo motor systems usually employ position detectors to enable the provision of the most effective position commands for driving the motor, and thereby to maximize operating efficiencies. The incorporation of such a device adds substantially to the cost and complexity of the motor system, however, and also increases the difficulty of utilizing it most effectively.
Stepping motor systems inherently afford high positioning performance and other beneficial attributes; because no position detection device is needed, moreover, they of course avoid the attendant disadvantages. On the other hand, practical operation of such motors requires the application of excessive drive currents so as to ensure that synchronism is maintained. Thus, and despite load changes, the current supplied must be set for the maximum that would normally be employed during acceleration; typically, the current utilized to drive a stepping motor is at least double that which is actually demanded by the load. Substantial amounts of energy are wasted (as excessive heat) as a result, thereby rendering such systems relatively inefficient in respect of power consumption.
An effective method and apparatus for detecting load torque in a stepping motor system, and for utilizing the same to optimize the current supplied for driving the motor, is described and claimed in Someya application Ser. No. 09/250,477, issued as U.S. Pat. No. 6,254,990 and of common assignment herewith. The technique described is directed however to systems in which pulse width modulated control of the drive current is employed, whereas many synchronous motors are operated under microstep control utilizing sinusoidal or similar current waveforms for energization of the phase windings.
Accordingly, the broad objects of the present invention are to provide a novel load torque estimating technique, a novel stepping or other synchronous motor system embodying the same, and a novel method and apparatus for controlling operation of such a motor, wherein and whereby drive current supplied to the motor can be optimized so as to improve energy utilization efficiency, without need for a position detector or similar means.
More specific objects of the invention are to provide such a system, apparatus, and method wherein and whereby the load torque upon the motor can be estimated and utilized to optimize (normally, by selective reduction) the current supplied for driving the motor.
Additional specific objects of the invention are to provide such a system, apparatus, and method wherein and whereby motor operation is relatively facile, and vibration, temperature rise, noise, complexity and cost are relatively low.
It has now been found that certain of the foregoing and related objects of the invention are attained by the provision of a method for estimating the load torque applied to the shaft of an operating synchronous motor, comprising the steps:
(a) establishing a current/torque relationship in which the integral of a drive current value, incorporating a current applied to drive the motor at a selected speed, varies as a function of the load torque on the shaft of the motor;
(b) driving the motor at the selected speed;
(c) measuring a current applied to drive the motor;
(d) calculating the integral of a drive current value that incorporates the measured current; and
(e) correlating the calculated integral to load torque, utilizing the established current/torque relationship, to thereby estimate the actual load torque applied to the shaft of the driven motor.
In carrying out the present method the measured drive current utilized will normally be a lead current (inclusive, of course, of phase currents). The current/torque relationship utilized will desirably be established by preliminary steps that include: (i) operating the motor at the selected speed under a plurality of torque load conditions; (ii) measuring the currents applied for driving the motor under each of the load conditions; and (iii) calculating integrals of drive current values incorporating the currents measured in step (ii).
Preferably, the drive current value of which the integral is calculated will also incorporate a linearizing reference signal, synchronized to the current applied to drive the motor and advantageously of square wave form, which reference signal causes the integral of the drive current value to vary as a substantially linear function of the load torque on the shaft of the motor. In such embodiments the procedure used to establish the current/torque relationship may desirably include preliminary steps in which the drive current values are calculated using an assumed value for the reference signal, the assumed value being varied as necessary to cause the integral calculated to vary as a substantially linear function of the load conditions and thereby to define the characteristics of the linearizing reference signal. In using such a procedure, the torque load conditions under which the drive currents employed in calculating the integral are measured will advantageously constitute only the no-load and the full-load (i.e., the load just below than that at which synchronism is lost) conditions, measurements under intermediate load conditions being unnecessary.
Other objects of the invention are attained by the provision of a method for driving a synchronous motor, comprising the steps:
I. establishing, as a target load factor, an optimal load torque that is to be produced by the motor operating at a selected speed, normally characterized as a proportion or percentage of the full-load torque of which the motor is capable at that speed;
II. establishing a current/torque relationship in which the integral of a drive current value, incorporating a current applied to drive the motor at the selected speed, varies as a function of the load torque on the shaft of the motor;
III. driving the motor at the selected speed; and
IV. iteratively, and in synchronism with the operating cycle of the motor, carrying out the following sequence of steps:
(a) measuring a current applied to drive the motor;
(b) calculating the integral of a drive current value that incorporates the measured current;
(c) correlating the calculated integral to the load torque, utilizing the established current/torque relationship, and thereby estimating the actual load torque applied to the shaft of the operating motor;
(d) calculating an estimated load ratio, normally constituting the ratio of the estimated actual load torque to the full-load torque for the motor operating at the selected speed;
(e) comparing the estimated load ratio to the target load factor; and
(f) adjusting the amplitude of the current applied to drive the motor as necessary to cause the estimated load ratio to approach the target load factor.
In implementing the foregoing motor-driving method, the estimated load ratio will desirably be calculated in accordance with the expression:
ELR=(Nxe2x88x92M)/(Nxe2x88x92F)
wherein ELR is the estimated load ratio, N is a calculated integral of the drive current value that incorporates a measured current applied to drive the motor under the no-load conditions, M is the integral calculated in step (b) above, and F is a calculated integral of the drive current value that incorporates a measured current applied to drive the motor under the full-load condition. (Needless to say, the product is multiplied by 100 if the ELR is to be expressed as a percentage value.)
Additional objects of the invention are attained by the provision of apparatus for controlling operation of a synchronous motor, comprising:
(a) means for storing electronically data defining the current/torque relationship described herein;
(b) means for measuring a current applied to drive the motor at a selected speed;
(c) means for calculating the integral of a drive current value that incorporates the measured current; and
(d) means for correlating load torque to the calculated integral, utilizing the stored current/torque relationship data, thereby to estimate the actual load torque applied to the shaft of a driven motor. Such control apparatus may further comprise:
(e) means for storing electronically the described target load factor; and
(f) means for carrying out, on a cyclical basis timed to the operating cycle of the motor, each of the steps:
(i) measuring a current applied to drive the motor;
(ii) calculating the integral of a drive current value that incorporates the current measured in step (i);
(iii) correlating the integral calculated in step (ii) to load torque, utilizing the stored current/torque relationship data, and thereby estimating the actual load torque applied to the shaft of the motor;
(iv) calculating an estimated load ratio, constituting the ratio of actual load torque estimated in (iii) to the full-load torque for the motor operating at a selected speed;
(v) comparing the estimated load ratio calculated in step (iv) to the stored target load factor; and
(vi) adjusting the amplitude of the current applied to drive the motor, as necessary to cause the calculated estimated load to approach the target load factor.
Yet additional objects are attained by the provision of a motor system comprised of a synchronous motor, control apparatus as described herein, and means operatively connecting the motor and the control apparatus to enable the control apparatus to control operation of the motor.